AD9480-LVDS-PCB3 AD [Analog Devices], AD9480-LVDS-PCB3 Datasheet - Page 14

• Manufacturer Part Number: AD9480-LVDS-PCB3
• Description: 8-Bit, 250 MSPS 3.3 V A/D Converter
• Manufacturer: AD [Analog Devices]

AD9480
External Reference
An external reference can be used for greater accuracy and
temperature stability when required. The gain of the AD9480
can also be varied using this configuration. A voltage output
DAC can be used to set VREF, providing for a means to digitally
adjust the full-scale voltage. VREF can be externally set to
voltages from .75 V to 1.5 V; optimum performance is typically
obtained at VREF = 1 V. (See the Typical Performance
Characteristics section.)
Programmable Reference
The programmable reference can be used to set a differential
input span anywhere between 0.75 V p-p and 1.5 V p-p by using
an external resistor divider. The sense pin will self-bias to 0.5 V,
and the resulting VREF is equal to 0.5 × (1 + R1/R2). It is
recommended to keep the sum of R1+R2 ≥ 10 kΩ to limit
VREF loading (for VREF=1.5 V, set R1 equal to 7 kΩ and R2
equal to 3.5 kΩ).
DIGITAL OUTPUTS
LVDS outputs are available when a 3.7 kΩ RSET resistor is
placed at Pin 42 (LVDSBIAS) to ground. The RSET resistor
current (~ 1.2 V/RSET) is ratioed on-chip setting the output
current at each output equal to a nominal 3.5 mA with an RSET
of 3.74 kΩ. Varying the RSET current also linearly changes the
LVDS output current, resulting in a variable output swing for a
fixed termination resistance.
A 100 Ω differential termination resistor placed at the LVDS
receiver inputs results in a nominal 350 mV swing at the
receiver. LVDS mode facilitates interfacing with LVDS receivers
in custom ASICs and FPGAs that have LVDS capability for
superior switching performance in noisy environments. Single
point-to-point net topologies are recommended with a 100 Ω
termination resistor as close to the receiver as possible. Keep the
trace length 3 to 4 inches maximum and the differential output
trace lengths as equal as possible.
REFERENCE OR
10µF
DAC INPUT
EXTERNAL
Figure 19. Programmable Reference
Figure 18. External Reference
0.1µF
MAY REQUIRE
RC FILTER
AVDD
R1
R2
VREF
SENSE
VREF
SENSE
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OUTPUT CODING
Table 10.
Code
255
255
254
•
•
129
128
127
•
•
2
1
0
0
INTERLEAVING TWO AD9480s
Instrumentation applications may prefer to interleave, or
ping-pong, two AD9480s to achieve twice the sample rate, or
500 MSPS. In these applications, it is important to match the
gain and offset of the two ADCs. Varying the reference voltage
allows the gain of the ADCs to be adjusted; external dc offset
compensation can be used to reduce offset mismatch between
two ADCs. The sampling phase offset between the two ADCs is
extremely important as well, and requires very low skew
between clock signals driving the ADCs (< 2 pS clock skew for a
100 MHz analog input frequency).
DATA CLOCK OUT
An LVDS data clock is available at DCO+ and DCO − . These
clocks can facilitate latching off-chip, providing a low skew
clocking solution. The on-chip delay of the DCO clocks tracks
with the on chip delay of the data bits, (under similar loading)
such that the variation between Tpd and Tcpd is minimized. It
is recommended to keep the trace lengths on the data and DCO
pins matched and to 3 to 4 inches maximum. The output and
DCO outputs should be designed for a differential characteristic
impedance of 100 Ω, and terminated differentially at the
receiver with 100 Ω.
(VIN+) − (VIN−)
> 0.512 V
0.512 V
0.508 V
•
•
0.004 V
0.0 V
–0.004 V
•
•
-0.504 V
–0.508 V
–0.512 V
< –0.512 V
Offset Binary
•
•
1000 0001
1000 0000
0111 1111
•
•
0000 0010
0000 0001
0000 0000
0000 0000
1111 1111
1111 1111
1111 1110
Twos Complement
0111 1111
0111 1111
0111 1110
•
•
0000 0001
0000 0000
1111 1111
•
•
1000 0010
1000 0001
1000 0000
1000 0000